Variable speed transmissions which permit the selection of any drive ratio offer the prospect of better efficiency by permitting a closer match between the most efficient engine speed with a given vehicle speed. Although providing this advantage, prior art transmissions designed to provide a variable ratio range have encountered substantial difficulties in the area of noise, premature wear and power handling limitations. In prior art belt drive variable transmission systems, slippage problems between the belt and the pulleys may be encountered under power. In the prior art units, slippage occurs at lower gear ratios, thus resulting in wear and loss of efficiency. This is particularly the case where the belt is susceptible to oil contamination. Although metal belts may be used, excessive wear and noise are an undesirable by-product of this substitution.
One prior art system, identified as a belt drive continuously variable transmission produced by Van Doorne Transmissie of Holland, provides variable effective diameter pulleys mounted on a driving and driven shaft. The pulleys have movable pulley halves which separate and close to change their effective diameters for an endless belt encircled therearound. To vary the "gearing" ratio between the drive and driven shafts from a low to high "gear", the drive pulley closes, forcing the belt to move higher in the effective V-shaped pulley groove. Simultaneously therewith, the driven pulley opens, permitting the belt to ride lower in the pulley groove, thus increasing gearing.
The use of variable effective diameter pulleys in variable speed transmissions is also shown in the transmissions disclosed in U.S. Pat. Nos. 3,214,997 and 2,970,493 to Rieser, U.S. Pat. No. 3,287,988 to Heyer and U.S. Pat. No. 3,924,480 to Carapellucci. In each of these arrangements, variable effective diameter pulleys are used with a continuous belt for engaging the pulleys in a similar fashion. Generally, the effective diameter of one pulley is increased while the effective diameter of the other is decreased to compensate for belt length. In one embodiment of the transmission disclosed in the reference to Rieser, U.S. Pat. No. 3,214,997, one variable effective diameter pulley is disclosed with belt tension being compensated for by movement of the pulley relative to the fixed diameter pulley.